import Foundation
import SwiftData
import UniformTypeIdentifiers
import Network
import CryptoKit

// Remote contract: Docs/RemoteAPI.openapi.yaml

struct RemoteAsset: Decodable {
    let id: String
    let clientId: String
    let kind: String
    let fileName: String
    let mimeType: String
    let sizeBytes: Int64
    let sha256: String
    let createdAt: Date?
}

struct AssetUploadResult {
    let asset: RemoteAsset
    let sessionRevision: Int64?
}

struct StorageQuota: Decodable {
    let totalBytes: Int64
    let usedBytes: Int64
    let remainingBytes: Int64
}

struct RemoteEvent: Decodable {
    let id: String
    let clientId: String?
    let relativeTimeMs: Int64
    let eventType: String
    let textContent: String?
    let voiceStartOffsetMs: Int64?
    let voiceEndOffsetMs: Int64?
}

struct RemoteSession: Decodable {
    let id: String
    let clientId: String?
    let title: String
    let startTime: Date
    let endTime: Date?
    let durationMs: Int64
    let revision: Int64
    let deletedAt: Date?
    let events: [RemoteEvent]?
    let assets: [RemoteAsset]?
}

private struct SessionUpload: Encodable {
    let clientId: String
    let title: String
    let startTime: Date
    let endTime: Date?
    let durationMs: Int64
    let events: [EventUpload]
    let baseRevision: Int64?
    let deletedEventClientIds: [String]
}

private struct EventUpload: Encodable {
    let clientId: String
    let relativeTimeMs: Int64
    let eventType: String
    let textContent: String?
    let voiceStartOffsetMs: Int64?
    let voiceEndOffsetMs: Int64?
}

private struct SyncCheckpoint: Decodable {
    let syncedAt: Date
}

private struct AssetUploadPayload: Decodable {
    let asset: RemoteAsset
    let sessionRevision: Int64?

    private enum CodingKeys: String, CodingKey {
        case asset
        case sessionRevision
    }

    init(from decoder: Decoder) throws {
        if let container = try? decoder.container(keyedBy: CodingKeys.self),
           container.contains(.asset) {
            asset = try container.decode(RemoteAsset.self, forKey: .asset)
            sessionRevision = try container.decodeIfPresent(Int64.self, forKey: .sessionRevision)
        } else {
            asset = try RemoteAsset(from: decoder)
            sessionRevision = nil
        }
    }
}

private struct ChunkUploadInitBody: Encodable {
    let clientId: String
    let kind: String
    let fileName: String
    let mimeType: String
    let fileSize: Int64
    let chunkSize: Int
}

private struct ChunkUploadInitResponse: Decodable {
    let uploadId: String
    let totalChunks: Int
    let chunkSize: Int
}

private struct ChunkUploadProgressResponse: Decodable {
    let uploadedChunks: Int
    let totalChunks: Int
}

private struct ChunkUploadCompleteBody: Encodable {
    let uploadId: String
    let totalChunks: Int
}

final class RemoteNetworkService: NetworkServiceProtocol {
    private static let chunkedUploadThreshold: Int64 = 16 * 1024 * 1024
    private static let preferredChunkSize = 8 * 1024 * 1024
    private let client: APIClient

    init(client: APIClient = .shared) {
        self.client = client
    }

    func fetchSessions() async throws -> [RemoteSession] {
        try await client.request("sessions?includeDeleted=true", authenticated: true)
    }

    func syncSession(
        _ session: CelestiaSession,
        deletedEventClientIDs: [String]
    ) async throws -> RemoteSession {
        let payload = SessionUpload(
            clientId: session.id.uuidString,
            title: session.title,
            startTime: session.startTime,
            endTime: session.endTime,
            durationMs: session.durationMs,
            events: session.events.map {
                EventUpload(
                    clientId: $0.id.uuidString,
                    relativeTimeMs: $0.relativeTimeMs,
                    eventType: $0.eventType,
                    textContent: $0.textContent,
                    voiceStartOffsetMs: $0.voiceStartOffsetMs,
                    voiceEndOffsetMs: $0.voiceEndOffsetMs
                )
            },
            baseRevision: session.serverRevision > 0 ? session.serverRevision : nil,
            deletedEventClientIds: deletedEventClientIDs
        )
        let encoder = JSONEncoder()
        encoder.dateEncodingStrategy = .iso8601
        return try await client.request("sessions", method: .post, body: try encoder.encode(payload), authenticated: true)
    }

    func fetchStorageQuota() async throws -> StorageQuota {
        try await client.request("storage/quota", authenticated: true)
    }

    func uploadAsset(
        sessionID: String,
        clientID: String,
        kind: String,
        fileURL: URL,
        progress: @escaping @Sendable (Double) -> Void
    ) async throws -> AssetUploadResult {
        let mimeType = UTType(filenameExtension: fileURL.pathExtension)?.preferredMIMEType ?? "application/octet-stream"
        let size = try fileSize(of: fileURL)
        if size >= Self.chunkedUploadThreshold {
            return try await uploadAssetInChunks(
                sessionID: sessionID,
                clientID: clientID,
                kind: kind,
                fileURL: fileURL,
                fileSize: size,
                mimeType: mimeType,
                progress: progress
            )
        }
        let payload: AssetUploadPayload = try await client.upload(
            "sessions/\(sessionID)/assets",
            fileURL: fileURL,
            fileName: fileURL.lastPathComponent,
            mimeType: mimeType,
            fields: ["clientId": clientID, "kind": kind],
            progress: progress
        )
        return AssetUploadResult(asset: payload.asset, sessionRevision: payload.sessionRevision)
    }

    func downloadAsset(sessionID: String, asset: RemoteAsset, destinationURL: URL) async throws {
        try await client.download("sessions/\(sessionID)/assets/\(asset.id)", to: destinationURL)
    }

    func recordSyncCheckpoint() async throws {
        let _: SyncCheckpoint = try await client.request("sync/trigger", method: .post, authenticated: true)
    }

    private func uploadAssetInChunks(
        sessionID: String,
        clientID: String,
        kind: String,
        fileURL: URL,
        fileSize: Int64,
        mimeType: String,
        progress: @escaping @Sendable (Double) -> Void
    ) async throws -> AssetUploadResult {
        let body = ChunkUploadInitBody(
            clientId: clientID,
            kind: kind,
            fileName: fileURL.lastPathComponent,
            mimeType: mimeType,
            fileSize: fileSize,
            chunkSize: Self.preferredChunkSize
        )
        let encoder = JSONEncoder()
        let upload: ChunkUploadInitResponse = try await client.request(
            "sessions/\(sessionID)/assets/init",
            method: .post,
            body: try encoder.encode(body),
            authenticated: true
        )

        for chunkIndex in 0..<upload.totalChunks {
            let offset = UInt64(chunkIndex) * UInt64(upload.chunkSize)
            let preparedChunk = try await prepareUploadChunk(
                fileURL: fileURL,
                uploadID: upload.uploadId,
                chunkIndex: chunkIndex,
                chunkSize: upload.chunkSize,
                offset: offset
            )

            let chunkStart = Double(offset) / Double(fileSize)
            let chunkSpan = Double(preparedChunk.byteCount) / Double(fileSize)
            do {
                let _: ChunkUploadProgressResponse = try await client.upload(
                    "sessions/\(sessionID)/assets/chunk",
                    fileURL: preparedChunk.url,
                    fileName: "chunk-\(chunkIndex)",
                    mimeType: "application/octet-stream",
                    fields: [
                        "uploadId": upload.uploadId,
                        "chunkIndex": String(chunkIndex)
                    ]
                ) { fraction in
                    progress(min(max(chunkStart + chunkSpan * fraction, 0), 1))
                }
                try? FileManager.default.removeItem(at: preparedChunk.url)
            } catch {
                try? FileManager.default.removeItem(at: preparedChunk.url)
                throw error
            }
        }

        let completeBody = ChunkUploadCompleteBody(
            uploadId: upload.uploadId,
            totalChunks: upload.totalChunks
        )
        let payload: AssetUploadPayload = try await client.request(
            "sessions/\(sessionID)/assets/complete",
            method: .post,
            body: try encoder.encode(completeBody),
            authenticated: true
        )
        progress(1)
        return AssetUploadResult(asset: payload.asset, sessionRevision: payload.sessionRevision)
    }

    private func prepareUploadChunk(
        fileURL: URL,
        uploadID: String,
        chunkIndex: Int,
        chunkSize: Int,
        offset: UInt64
    ) async throws -> (url: URL, byteCount: Int) {
        let task = Task.detached(priority: .utility) {
            try Task.checkCancellation()
            let input = try FileHandle(forReadingFrom: fileURL)
            defer { try? input.close() }
            try input.seek(toOffset: offset)
            guard let data = try input.read(upToCount: chunkSize), !data.isEmpty else {
                throw APIError.transport("读取上传分片失败")
            }
            try Task.checkCancellation()
            let temporaryURL = FileManager.default.temporaryDirectory
                .appendingPathComponent("celestia-\(uploadID)-\(chunkIndex).chunk")
            try data.write(to: temporaryURL, options: .atomic)
            return (temporaryURL, data.count)
        }
        return try await withTaskCancellationHandler {
            try await task.value
        } onCancel: {
            task.cancel()
        }
    }

    private func fileSize(of url: URL) throws -> Int64 {
        let values = try url.resourceValues(forKeys: [.fileSizeKey])
        guard let size = values.fileSize else {
            throw APIError.transport("无法读取待上传文件大小")
        }
        return Int64(size)
    }
}

struct RemoteBoundDevice: Decodable {
    let id: String
    let name: String
    let peripheralUUID: String
}

private struct BindDeviceBody: Encodable {
    let name: String
    let peripheralUUID: String
    let hardwareMAC: String?
    let firmwareVersion: String?
    let batteryLevel: Int?
    let freeStorageMB: Int?
    let totalStorageMB: Int?
}

private struct UpdateDeviceBody: Encodable {
    let name: String?
    let batteryLevel: Int?
    let isConnected: Bool?
    let firmwareVersion: String?
    let freeStorageMB: Int?
    let totalStorageMB: Int?
}

final class DeviceCloudService {
    static let shared = DeviceCloudService()
    private let client: APIClient

    init(client: APIClient = .shared) {
        self.client = client
    }

    func register(_ device: BoundDevice) async throws -> RemoteBoundDevice {
        let body = BindDeviceBody(
            name: device.name,
            peripheralUUID: device.peripheralUUID,
            hardwareMAC: device.hardwareMAC,
            firmwareVersion: device.firmwareVersion,
            batteryLevel: device.batteryLevel,
            freeStorageMB: device.freeStorageMB,
            totalStorageMB: device.totalStorageMB
        )
        return try await client.request("devices", method: .post, body: try JSONEncoder().encode(body), authenticated: true)
    }

    func update(_ device: BoundDevice) async throws -> RemoteBoundDevice {
        guard let cloudID = device.cloudID else { return try await register(device) }
        let body = UpdateDeviceBody(
            name: device.name,
            batteryLevel: device.batteryLevel,
            isConnected: device.isConnected,
            firmwareVersion: device.firmwareVersion,
            freeStorageMB: device.freeStorageMB,
            totalStorageMB: device.totalStorageMB
        )
        return try await client.request("devices/\(cloudID)", method: .put, body: try JSONEncoder().encode(body), authenticated: true)
    }

    func remove(cloudID: String) async throws {
        try await client.requestVoid("devices/\(cloudID)", method: .delete, authenticated: true)
    }
}

final class NetworkStatusMonitor: ObservableObject, @unchecked Sendable {
    static let shared = NetworkStatusMonitor()

    @Published private(set) var isConnected = true
    @Published private(set) var isWiFi = false
    @Published private(set) var isCellular = false

    private let monitor = NWPathMonitor()
    private let queue = DispatchQueue(label: "com.celestia.trace.network-path")

    private init() {
        monitor.pathUpdateHandler = { [weak self] path in
            let connected = path.status == .satisfied
            let usesWiFi = path.usesInterfaceType(.wifi)
            let usesCellular = path.usesInterfaceType(.cellular)
            DispatchQueue.main.async {
                self?.isConnected = connected
                self?.isWiFi = usesWiFi
                self?.isCellular = usesCellular
            }
        }
        monitor.start(queue: queue)
    }

    var connectionName: String {
        if isWiFi { return "Wi-Fi" }
        if isCellular { return "蜂窝网络" }
        return isConnected ? "当前网络" : "网络"
    }
}

@MainActor
final class SyncManager: ObservableObject {
    static let shared = SyncManager()

    @Published private(set) var isSyncing = false
    @Published private(set) var lastSyncDate: Date?
    @Published private(set) var lastErrorMessage: String?
    @Published private(set) var completedCount = 0
    @Published private(set) var totalCount = 0
    @Published private(set) var activeSessionID: UUID?
    @Published private(set) var syncProgress: Double = 0

    private let service: NetworkServiceProtocol
    private var activeSyncTask: Task<Bool, Never>?
    private var automaticSyncTasks: [UUID: Task<Void, Never>] = [:]
    private var automaticSyncGenerations: [UUID: UUID] = [:]

    init(service: NetworkServiceProtocol = RemoteNetworkService()) {
        self.service = service
        lastSyncDate = UserDefaults.standard.object(forKey: "com.celestia.trace.last_server_sync") as? Date
    }

    func estimatedUploadBytes(for session: CelestiaSession) -> Int64 {
        var urls: [URL] = []
        if let url = AudioPathHelper.resolveURL(for: session.localAudioPath) {
            urls.append(url)
        }
        urls.append(contentsOf: session.events.compactMap { event in
            guard event.eventType == "PHOTO" else { return nil }
            return AudioPathHelper.resolveURL(for: event.localFilePath)
        })
        return urls.reduce(into: 0) { total, url in
            let values = try? url.resourceValues(forKeys: [.fileSizeKey])
            total += Int64(values?.fileSize ?? 0)
        }
    }

    func scheduleAutomaticSync(
        for session: CelestiaSession,
        modelContext: ModelContext,
        delay: Duration = .seconds(1.5)
    ) {
        guard session.endTime != nil,
              session.syncState != .conflict,
              AuthManager.shared.currentUser?.id != nil else { return }

        let sessionID = session.id
        let generation = UUID()
        automaticSyncGenerations[sessionID] = generation
        automaticSyncTasks[sessionID]?.cancel()
        if isSyncing, activeSessionID == sessionID {
            activeSyncTask?.cancel()
        }
        automaticSyncTasks[sessionID] = Task { @MainActor [weak self] in
            do {
                try await Task.sleep(for: delay)
                while let self,
                      (self.isSyncing || !NetworkStatusMonitor.shared.isConnected) {
                    try Task.checkCancellation()
                    try await Task.sleep(for: .seconds(0.75))
                }
                guard let self,
                      !Task.isCancelled,
                      self.automaticSyncGenerations[sessionID] == generation,
                      session.needsCloudSync,
                      let userID = AuthManager.shared.currentUser?.id else { return }
                _ = await self.sync(
                    sessions: [session],
                    modelContext: modelContext,
                    userID: userID
                )
                if self.automaticSyncGenerations[sessionID] == generation {
                    self.automaticSyncTasks[sessionID] = nil
                    self.automaticSyncGenerations[sessionID] = nil
                }
            } catch {
                guard let self,
                      self.automaticSyncGenerations[sessionID] == generation else { return }
                self.automaticSyncTasks[sessionID] = nil
                self.automaticSyncGenerations[sessionID] = nil
            }
        }
    }

    func sync(
        sessions: [CelestiaSession],
        modelContext: ModelContext,
        userID: String
    ) async -> Bool {
        guard !isSyncing else { return false }
        isSyncing = true
        let task = Task { @MainActor [self] in
            await performSync(
                sessions: sessions,
                modelContext: modelContext,
                userID: userID
            )
        }
        activeSyncTask = task
        let result = await withTaskCancellationHandler {
            await task.value
        } onCancel: {
            task.cancel()
        }
        activeSyncTask = nil
        isSyncing = false
        activeSessionID = nil
        return result
    }

    func pauseSync(sessionID: UUID) {
        guard isSyncing, activeSessionID == sessionID else { return }
        activeSyncTask?.cancel()
    }

    private func performSync(
        sessions: [CelestiaSession],
        modelContext: ModelContext,
        userID: String
    ) async -> Bool {
        lastErrorMessage = nil
        completedCount = 0
        activeSessionID = nil
        syncProgress = 0
        let queuedSessions = sessions.filter {
            ($0.ownerUserID == nil || $0.ownerUserID == userID)
                && $0.needsCloudSync
                && $0.syncState != .conflict
        }
        totalCount = queuedSessions.count
        // Give the UI immediate feedback while the remote index is being fetched.
        // The persisted state is changed only when this session actually begins syncing.
        activeSessionID = queuedSessions.first?.id
        do {
            let remoteSessions = try await service.fetchSessions()
            try Task.checkCancellation()
            let mergedSessions = merge(remoteSessions, into: sessions, modelContext: modelContext, userID: userID)
            for (remote, local) in mergedSessions {
                try await downloadMissingAssets(for: local, remote: remote)
                try Task.checkCancellation()
            }
            try modelContext.save()

            let eligible = sessions.filter {
                ($0.ownerUserID == nil || $0.ownerUserID == userID)
                    && $0.needsCloudSync
                    && $0.syncState != .conflict
            }
            var failures: [String] = []
            let eligibleCount = max(eligible.count, 1)
            for (index, session) in eligible.enumerated() {
                activeSessionID = session.id
                let sessionStartProgress = Double(index) / Double(eligibleCount)
                let sessionProgressSpan = 1.0 / Double(eligibleCount)
                syncProgress = sessionStartProgress
                session.ownerUserID = userID
                session.syncState = .syncing
                session.lastSyncError = nil
                do {
                    let previousRemote = remoteSessions.first {
                        $0.id == session.cloudSessionId
                            || $0.clientId?.caseInsensitiveCompare(session.id.uuidString) == .orderedSame
                    }
                    let localEventIDs = Set(session.events.map { $0.id.uuidString.lowercased() })
                    let deletedEventClientIDs = previousRemote?.events?
                        .compactMap(\.clientId)
                        .filter { !localEventIDs.contains($0.lowercased()) } ?? []
                    let remote = try await service.syncSession(
                        session,
                        deletedEventClientIDs: deletedEventClientIDs
                    )
                    try Task.checkCancellation()
                    syncProgress = sessionStartProgress + sessionProgressSpan * 0.12
                    session.cloudSessionId = remote.id
                    session.serverRevision = remote.revision
                    try await uploadLocalAssets(for: session, remote: remote) { [weak self] assetProgress in
                        Task { @MainActor [weak self] in
                            self?.syncProgress = sessionStartProgress
                                + sessionProgressSpan * (0.12 + assetProgress * 0.83)
                        }
                    }
                    try Task.checkCancellation()
                    session.isSynced = true
                    session.syncState = .synced
                    session.lastSyncedAt = Date()
                    completedCount += 1
                    syncProgress = sessionStartProgress + sessionProgressSpan
                } catch where Task.isCancelled {
                    session.isSynced = false
                    session.syncState = .pending
                    session.lastSyncError = nil
                    syncProgress = 0
                    try? modelContext.save()
                    return false
                } catch APIError.server(let code, _) where code == 409 {
                    session.isSynced = false
                    session.syncState = .conflict
                    session.lastSyncError = "本地版本 \(session.serverRevision) 与云端版本不一致"
                } catch {
                    session.isSynced = false
                    session.syncState = .failed
                    session.lastSyncError = error.localizedDescription
                    failures.append("\(session.title)：\(error.localizedDescription)")
                }
                try modelContext.save()
            }
            let conflicts = sessions.filter { $0.syncState == .conflict }
            if !conflicts.isEmpty {
                failures.append(contentsOf: conflicts.map {
                    "\($0.title)：本地和云端版本不一致"
                })
            }
            guard failures.isEmpty else {
                lastErrorMessage = failures.joined(separator: "\n")
                return false
            }
            try await service.recordSyncCheckpoint()
            let now = Date()
            lastSyncDate = now
            UserDefaults.standard.set(now, forKey: "com.celestia.trace.last_server_sync")
            return true
        } catch where Task.isCancelled {
            syncProgress = 0
            return false
        } catch {
            lastErrorMessage = error.localizedDescription
            return false
        }
    }

    private func uploadLocalAssets(
        for session: CelestiaSession,
        remote: RemoteSession,
        progress: @escaping @Sendable (Double) -> Void
    ) async throws {
        guard let cloudID = session.cloudSessionId else { throw APIError.missingData }
        let existingIDs = Set((remote.assets ?? []).map(\.clientId))
        var uploads: [(clientID: String, kind: String, url: URL, size: Int64)] = []
        if let path = session.localAudioPath {
            guard let url = AudioPathHelper.resolveURL(for: path) else {
                throw APIError.transport("本地录音文件不存在：\(path)")
            }
            let sha256 = try await fileSHA256(of: url)
            let hasMatchingAudio = (remote.assets ?? []).contains {
                $0.kind.uppercased() == "AUDIO"
                    && $0.sha256.caseInsensitiveCompare(sha256) == .orderedSame
            }
            if !hasMatchingAudio {
                let clientID = "\(session.id.uuidString)-audio-\(sha256.prefix(16))"
                uploads.append((clientID, "AUDIO", url, fileSize(of: url)))
            }
        }
        for event in session.events where event.eventType == "PHOTO" {
            guard let path = event.localFilePath else { continue }
            guard let url = AudioPathHelper.resolveURL(for: path) else {
                throw APIError.transport("照片文件不存在：\(path)")
            }
            let clientID = "\(event.id.uuidString)-photo"
            if !existingIDs.contains(clientID) {
                uploads.append((clientID, "PHOTO", url, fileSize(of: url)))
            }
        }

        let totalBytes = max(uploads.reduce(Int64(0)) { $0 + $1.size }, 1)
        let requiredBytes = uploads.reduce(Int64(0)) { $0 + $1.size }
        if requiredBytes > 0 {
            let quota = try await service.fetchStorageQuota()
            guard requiredBytes <= quota.remainingBytes else {
                throw APIError.storageQuotaExceeded(
                    requiredBytes: requiredBytes,
                    remainingBytes: quota.remainingBytes
                )
            }
        }
        var completedBytes: Int64 = 0
        progress(uploads.isEmpty ? 1 : 0)
        for upload in uploads {
            let bytesBeforeUpload = completedBytes
            let result = try await service.uploadAsset(
                sessionID: cloudID,
                clientID: upload.clientID,
                kind: upload.kind,
                fileURL: upload.url
            ) { fraction in
                let sentBytes = Double(bytesBeforeUpload) + Double(upload.size) * fraction
                progress(min(max(sentBytes / Double(totalBytes), 0), 1))
            }
            if let revision = result.sessionRevision {
                session.serverRevision = revision
            }
            completedBytes += upload.size
            progress(Double(completedBytes) / Double(totalBytes))
        }
    }

    private func fileSize(of url: URL) -> Int64 {
        let values = try? url.resourceValues(forKeys: [.fileSizeKey])
        return Int64(values?.fileSize ?? 0)
    }

    private func fileSHA256(of url: URL) async throws -> String {
        let task = Task.detached(priority: .utility) {
            let input = try FileHandle(forReadingFrom: url)
            defer { try? input.close() }
            var hasher = SHA256()
            while let data = try input.read(upToCount: 1_024 * 1_024),
                  !data.isEmpty {
                try Task.checkCancellation()
                hasher.update(data: data)
            }
            return hasher.finalize().map { String(format: "%02x", $0) }.joined()
        }
        return try await withTaskCancellationHandler {
            try await task.value
        } onCancel: {
            task.cancel()
        }
    }

    private func downloadMissingAssets(for session: CelestiaSession, remote: RemoteSession) async throws {
        let assets = remote.assets ?? []
        if let audio = assets
            .filter({ $0.kind.uppercased() == "AUDIO" })
            .max(by: {
                ($0.createdAt ?? .distantPast) < ($1.createdAt ?? .distantPast)
            }) {
            let localURL = AudioPathHelper.resolveURL(for: session.localAudioPath)
            let localHash: String?
            if let localURL {
                localHash = try? await fileSHA256(of: localURL)
            } else {
                localHash = nil
            }
            let needsDownload = localURL == nil
                || (session.isSynced
                    && localHash?.caseInsensitiveCompare(audio.sha256) != .orderedSame)
            if needsDownload {
                let destination = downloadDestination(for: audio)
                try await service.downloadAsset(sessionID: remote.id, asset: audio, destinationURL: destination)
                session.localAudioPath = AudioPathHelper.relativePath(from: destination.path)
            }
        }

        let photoAssets = Dictionary(uniqueKeysWithValues: assets
            .filter { $0.kind.uppercased() == "PHOTO" }
            .map { ($0.clientId.lowercased(), $0) })
        for event in session.events where event.eventType == "PHOTO" {
            guard event.localFilePath == nil || AudioPathHelper.resolveURL(for: event.localFilePath) == nil else { continue }
            let key = "\(event.id.uuidString)-photo".lowercased()
            guard let asset = photoAssets[key] else { continue }
            let destination = downloadDestination(for: asset)
            try await service.downloadAsset(sessionID: remote.id, asset: asset, destinationURL: destination)
            event.localFilePath = AudioPathHelper.relativePath(from: destination.path)
        }
    }

    private func downloadDestination(for asset: RemoteAsset) -> URL {
        let documents = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        let fileExtension = (asset.fileName as NSString).pathExtension
        let suffix = fileExtension.isEmpty ? "" : ".\(fileExtension.lowercased())"
        return documents.appendingPathComponent("cloud_\(asset.id)\(suffix)")
    }

    private func merge(
        _ remoteSessions: [RemoteSession],
        into localSessions: [CelestiaSession],
        modelContext: ModelContext,
        userID: String
    ) -> [(RemoteSession, CelestiaSession)] {
        var byClientID = Dictionary(uniqueKeysWithValues: localSessions.map { ($0.id.uuidString.lowercased(), $0) })
        var merged: [(RemoteSession, CelestiaSession)] = []
        for remote in remoteSessions {
            guard let clientID = remote.clientId?.lowercased() else { continue }
            if let local = byClientID[clientID] {
                if remote.deletedAt != nil, local.isSynced {
                    modelContext.delete(local)
                    continue
                }
                if !local.isSynced,
                   remote.revision > local.serverRevision {
                    local.syncState = .conflict
                    local.lastSyncError = "本地版本 \(local.serverRevision) 与云端版本 \(remote.revision) 不一致"
                    continue
                }
                if local.isSynced, remote.revision > local.serverRevision {
                    apply(remote, to: local, userID: userID)
                }
                if remote.deletedAt == nil {
                    merged.append((remote, local))
                }
            } else if remote.deletedAt == nil {
                let id = UUID(uuidString: clientID) ?? UUID()
                let session = CelestiaSession(id: id, title: remote.title, startTime: remote.startTime)
                apply(remote, to: session, userID: userID)
                modelContext.insert(session)
                byClientID[clientID] = session
                merged.append((remote, session))
            }
        }
        try? modelContext.save()
        return merged
    }

    private func apply(_ remote: RemoteSession, to local: CelestiaSession, userID: String) {
        local.title = remote.title
        local.startTime = remote.startTime
        local.endTime = remote.endTime
        local.cloudSessionId = remote.id
        local.ownerUserID = userID
        local.serverRevision = remote.revision
        local.isSynced = true
        local.syncState = .synced
        local.lastSyncError = nil
        local.events.removeAll()
        for remoteEvent in remote.events ?? [] {
            let id = remoteEvent.clientId.flatMap(UUID.init(uuidString:)) ?? UUID()
            let event = CelestiaTimelineEvent(id: id, relativeTimeMs: remoteEvent.relativeTimeMs, eventType: remoteEvent.eventType)
            event.textContent = remoteEvent.textContent
            event.voiceStartOffsetMs = remoteEvent.voiceStartOffsetMs
            event.voiceEndOffsetMs = remoteEvent.voiceEndOffsetMs
            local.events.append(event)
        }
    }
}